Flexible substrate having interlaminar junctions, and process for producing the same

ABSTRACT

A process for producing a flexible substrate comprising of a film, an insulating resin layer, and a wiring pattern, said process comprising the steps of: (a) preparing a sheet member comprising, (i) the film, (ii) the insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face, and (iii) a front-sided wiring pattern embedded in said insulating resin layer formed on said front face of said film, and a rear-sided wiring pattern embedded in the insulating resin layer formed on said rear face of said film; and (b) pressing a part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern into the inside of said sheet member so that a part of said front-sided wiring pattern and a part of said rear-sided wiring pattern are jointed to each other to form a junction.

FIELD OF THE INVENTION

The present invention relates to a flexible substrate, and a process for producing the same. In particular, the present invention relates to a flexible substrate and a multilayer flexible substrate in which a part of a front-sided wiring pattern and a part of a rear-sided wiring pattern are jointed to each other in such a manner that they penetrate through a film, and also relates to a process for producing the same.

BACKGROUND OF THE INVENTION

Recently, not only is electronics device becoming smaller, lighter and thinner, but also the electronic circuit is becoming high-speed processing. Therefore, it has been considerably required to achieve a miniaturization and a high frequency-wave performance of the electronic components. For example, as to a portable electronics device such as a cellular phone, it is one of the most important challenges to achieve a smaller, lighter and thinner device (see Japanese Patent Kokai Publication No. 2003-163422, for example). Accordingly, it is needed that a miniaturization and a high frequency-wave performance are achieved by mounting various types of the mounted-components with a short length of the wiring at a high density.

In these circumstances, a flexible substrate which leads to achievement of a high-density mounting is getting so much attention (see Japanese Patent Kokai Publication No. 2001-111189, for example). Hereinafter, a process for producing a conventional flexible substrate will be described with respect to FIG. 1.

First, as shown in FIG. 1(a), by means of a drill or a laser machining, holes 502 for interlaminar connections are formed in a insulating sheet 501 having about 50 to 100 μm in thickness. Next, as shown in FIG. 1(b), the holes 502 are filled with a conductive paste 503 by means of a printing method. Subsequently, as shown in FIG. 1(c), metal foils (i.e. copper foils) 504 are disposed on both surfaces of the insulating sheet 501, and thereafter the metal foils 504 are pressed in order to be stacked as shown in FIG. 1(d). After that, as shown in FIG. (e), resist films 505 having the same pattern as conductor circuits are formed on the metal foils 504. Subsequently, by using the resist films 505 as etch masks, a part of the metal foils 504 is etched away, followed by removing the resist films 505. As a result of that, a flexible substrate 500 having conductor circuits 506 is obtained as shown in FIG. 1(f).

In the next place, with respect to FIG. 2, a process for producing a conventional multilayer flexible substrate 550 will be hereinafter described.

First, as shown in FIG. 2(a), by using the flexible substrate 500 of FIG. (f) as a core substrate, insulating sheet 501 a in which holes 502 thereof are filled with a conductive paste as well as metal foils 504 a are stacked on that flexible substrate 500 in order to obtain the stacked substrate. Next, as shown in FIG. 2(b), resist films 505 are selectively formed on both surfaces of the stacked substrate. Subsequently, by using resist films 505 as etch masks, a part of the metal foils 504 a is etched away, followed by removing the resist films 505. As a result of that, a multilayer flexible substrate 550 composed of four-layer conductor circuits is obtained as shown in FIG. 2(c).

As described above, an etching technique (i.e. a wet process) is employed for the purpose of forming the wire pattern in the case of the conventional production process. Therefore, an influence of an etchant on an insulating sheet is of concern. In this case, it was troublesome to carry out the washing and drying steps as a post-treatment. Furthermore, the conventional wiring patterns formed by the etching technique are exposed to their surroundings on the surfaces of a flexible substrate. This will cause a microcrack of the wiring patterns when the flexible substrate is folded, which will be far from satisfying in terms of a flexing life.

Considering that the conventional process comprises the step for forming through-holes and thereafter filling the through-holes with a conductive paste, such conventional production process is fundamentally the same as a process for producing a rigid substrate (i.e. typical print circuit). The above-mentioned step is cumbersome (because of taking about 3 hours), so it is desired to simplify or abbreviate it. However, it has been considered that such step is essential for producing the flexible substrate as well as the multilayer flexible substrate, and that it is therefore basically difficult to abbreviate it. Also, due to an essential step, such step has been regarded as a matter of no concern. Therefore, there is no process for producing a flexible substrate and a multilayer flexible substrate with careful regard to the issues as described above.

Therefore, an object of the present invention is to provide a process for producing a flexible substrate and a multilayer flexible layer wherein a formation of through-holes and a filling of a conductive paste are abbreviated. Also, a further object of the present invention is to provide a flexible substrate and a multilayer flexible substrate as obtained by such process.

SUMMARY OF THE INVENTION

In order to achieve the object, the present invention provides a process (referred to also as “production process (I)” for producing a flexible substrate comprising of a film, an insulating resin layer and a wiring pattern, said process comprising the steps of:

-   -   (a) preparing a sheet member comprising,         -   (i) the film,         -   (ii) the insulating resin layer (i.e. electrically             insulating resin layer) formed on each of a front face of             said film and a rear face of said film which face is             opposite to said front face, and         -   (iii) a front-sided wiring pattern embedded in said             insulating resin layer formed on said front face of said             film, and a rear-sided wiring pattern embedded in the             insulating resin layer formed on said rear face of the film;             and     -   (b) pressing a part of at least one of said front-sided wiring         pattern and said rear-sided wiring pattern into the inside of         said sheet member so that a part of said front-sided wiring         pattern and a part of said rear-sided wiring pattern are jointed         to each other to form a junction. In addition, the present         invention provides a flexible substrate obtained by such process         (i.e. production process (I)).

Also, the present invention provides a process (referred to also as “production process (II)”) for producing a flexible substrate comprising of a sheet member and a substrate, said process comprising the steps of;

-   -   (a₁) preparing the sheet member comprising,         -   (i) a film,         -   (ii) an insulating resin layer formed on each of a front             face of said film and a rear face of said film which face is             opposite to said front face, and         -   (iii) a wiring pattern embedded in the insulating resin             layer formed on the front face of said film     -   (a₂) preparing the substrate having a wiring pattern formed on a         front face thereof,     -   (b) stacking said sheet member on said substrate in such a         manner that the insulating resin layer formed on said rear face         of said film of said sheet member is contacted with said front         face of said substrate, and thereafter a part of the wiring         pattern of said sheet member is pressed toward said substrate so         that said part of the wiring pattern of said sheet member and a         part of the wiring pattern of said substrate are jointed to each         other to form a junction. In addition, the present invention         provides a flexible substrate obtained by such process (i.e.         production process (II)).

Furthermore, the present invention provides a process for producing a multilayer flexible substrate wherein the step of the production process (I) and/or production process (II) as well as the step of for stacking further another flexible substrates is included. In addition, the present invention provides a multilayer flexible substrate obtained by such process.

The “junction (i.e. junction section)”, which is formed due to the joint or dent, serves to electrically connect the front-sided wiring pattern and the rear-sided wiring pattern to each other. Therefore, the term “junction” is herein referred to also as “interlaminar junction”.

As to a connection of the wiring patterns, the production processes (I) and (II) does not require a formation of through-holes and a filling of a conductive paste. According to the present invention, wiring patterns may be connected to each other by means of a needle-like member or a roll member with protrusions, for example. In the case where the roll member with a plurality of protrusions is used as a pair of roll members, a plurality of interlaminar junctions can be formed when a sheet member passes through a pair of roll members. Therefore, it is possible for production processes (I) and (II) to employ a roll-to-roll process, which in turn leads to a improved producibility and a mass production. This roll-to-roll process has an advantage of holding a sheet member easily while producing a flexible substrate.

Unlike in the case of filling with a conductive paste, an interlaminar junction prevents a discordance of an impedance between wiring patterns and vias (corresponding to the interlaminar junctions of the present invention) because the interlaminar junction consists of the same material as the wiring patterns in a seamless state. Also, due to the same material as the wiring patterns, there is no difference between a thermal expansion coefficient of each of the interlaminar junctions and that of the wiring patterns, which will lead to a better reliability in connection.

Furthermore, a sheet members used in production processes (I) and (II) can be obtained by transferring a wiring pattern which is preliminarily formed on a carrier sheet to each insulating resin layer formed on the film. Thus, not by means of a wet etching process but by means of a dry process, the sheet members can be prepared. In addition, a dry process can be carried out for the purpose of obtaining the interlaminar junction by using of a needle-like member or a roll member. Therefore, as a whole, the production processes (I) and (II) can be carried out by means of the dry process, which in turn leads to a simple production process that is easy to handle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to (f) show cross-sectional views illustrating the steps in a process for producing a conventional flexible substrate.

FIGS. 2(a) to (c) show cross-sectional views illustrating the steps in a process for producing a conventional flexible substrate.

FIG. 3 shows a cross-sectional view of a construction of a sheet member 10 used in the production process (I) of the present invention.

FIG. 4 shows a cross-sectional view of a construction of a flexible substrate obtained by the production process (I) of the present invention.

FIG. 5 shows a perspective view of a construction of a flexible substrate 100 obtained by the production process (I) of the present invention.

FIGS. 6(a) and (c) show cross-sectional views illustrating the steps in a process for preparing a sheet member 10.

FIG. 7 shows a schematic cross-sectional view of a transferring technique.

FIG. 8 shows a perspective view of a roll member 33 with protrusions 35.

FIG. 9 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced by means of a needle-like member 50.

FIG. 10 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced through the intermediary of conductive projecting members 27.

FIGS. 11(a) and (b) show cross-sectional views of embodiments wherein concave portions formed in a surface of a wiring pattern is filled with conductive members 27.

FIG. 12 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced by means of a needle-like member 50 as well as through the intermediary of a conductive projecting member 27.

FIG. 13 shows a perspective view of a roll member 34 without protrusions.

FIG. 14 shows a cross-sectional view of an embodiment wherein a conductive projecting member 27 is formed by means of an electrophotography technique.

FIG. 15 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced by disposing solders 15 between a film 11 and wiring patterns 20.

FIG. 16 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced through the intermediary of conductive projecting members 27 as well as solders 15.

FIG. 17 shows a cross-sectional view of an embodiment wherein a welding appliance 60 is used.

FIG. 18 shows a cross-sectional view of an embodiment wherein an ultrasonic wave is applied to junctions 25.

FIG. 19 shows a cross-sectional view of an embodiment for a production of a flexible substrate 100 wherein conductive members 15 are disposed on the inner side of a wiring pattern 20.

FIG. 20 shows a cross-sectional view of a construction of a multilayer flexible substrate 150 including a flexible substrate 100.

FIG. 21 shows cross-sectional view illustrating the steps in a process for producing a flexing substrate 100.

FIG. 22 shows a cross-sectional view illustrating the steps in a process for producing a flexing substrate 100 through the intermediary of conductive projecting members 27.

FIG. 23 shows a cross-sectional view of an embodiment wherein convex portions 26 are formed on a wiring pattern 20 by means of a roll member 36 with concave portions 35′.

FIG. 24 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced by means of a roll member 37 with protrusions 35 and concave portions 35′.

FIG. 25 shows a cross-sectional view of an embodiment wherein a flexible substrate 100 is produced by means of a roll-to-roll process.

FIG. 26 shows a cross-sectional view of an embodiment wherein a multilayer flexible substrate 150 is produced by means of a roll-to-roll process.

FIG. 27 shows a cross-sectional view of an embodiment wherein a transferring step of a wiring pattern 20 and a pressure-joint step are concurrently carried out.

FIG. 28 shows a cross-sectional view of a construction of a flexible substrate 200 obtained by a production process (II) of the present invention.

FIG. 29 shows a cross-sectional view of a construction of a flexible substrate 200 in which concave portions thereof are filled with conductive members 27.

FIG. 30 shows a perspective view of an example of a multilayer flexible substrate 205.

FIG. 31 shows a cross-sectional view illustrating the steps in a process for producing a multilayer flexible substrate 220 including a flexible substrate 200 of the present invention.

FIG. 32 shows a cross-sectional view of an embodiment of a transferring technique.

FIG. 33 shows a cross-sectional view illustrating the steps in a process for producing a multilayer flexible substrate 230 wherein sheet members 210 are stacked on both sides of a substrate 215.

FIG. 34 shows a cross-sectional view of an embodiment wherein a pressure-joint step is carried out by means of a roll member 33 with protrusions 35.

FIGS. 35(a) and (b) show cross-sectional views illustrating the steps in a process for producing a multilayer flexible substrate 250 wherein convex portions 26 are formed on a wiring pattern 17.

FIG. 36 shows a cross-sectional view illustrating the steps in a process for producing a multilayer flexible substrate 260 by using of a flexible substrate 100.

FIG. 37 shows a perspective view of a construction of a multilayer flexible substrate 260 obtained by a production process shown in FIG. 36.

FIG. 38 shows a perspective view of a construction of a multilayer flexible substrate 270 obtained by a production process (II).

FIG. 39 shows a perspective view of a construction of a multilayer flexible substrate 270 wherein only wiring patterns 17, 20 are seen through.

FIG. 40 shows a cross-sectional view of a construction of a flexible device 300.

FIG. 41 shows a cross-sectional view of a construction of a flexible substrate 130 including a flat metal layer.

FIG. 42 shows a cross-sectional view of a flexible device 300 including a composite sheet 84.

FIG. 43 shows a cross-sectional view of a construction of a flexible device 300 including electronic components.

FIGS. 44(a) and (b) show cross-sectional views illustrating the step for disposing passive components 85 a,85 b within a flexible device.

FIG. 45 shows a perspective view of a construction of an electronics device 400 in which a flexible substrate 100 is mounted as a circuit board.

FIG. 46 is a graph that shows an elongation modulus of a film versus a flexing number.

FIG. 47 is a graph that shows a curvature radius versus a flexing number.

FIG. 48 is a graph that shows a ratio of an insulating resin layer thickness/film thickness versus a flexing number.

FIG. 49 is a graph that shows a pressing load versus a resistance.

Hereinafter, the processes for producing a flexible substrate and a multilayer flexible substrate will be concretely described. In conjunction with that, the flexible substrate and the multilayer flexible substrate obtained by such processes will be also described concretely.

First of all, the production process (I) of the present invention will be described. In FIG. (3), a cross-sectional view of a construction of a sheet member 10 used in the step (a) is shown. Also, in FIGS. (4) and (5), cross-sectional and perspective views of a construction of the obtained flexible substrate 100 are respectively shown.

As for the step (a), a sheet member 10 is prepared. As shown in FIG. 3, the sheet member 10 comprises:

-   -   (i) the film 11;     -   (ii) the insulating resin layers 12 formed on a front face and a         rear face opposite to the front face of the film 11; and     -   (iii) the wiring patterns 20 embedded into the insulating resin         layers 12.

It is preferred that the film 11 is thinner than the insulating resin layer 12. For example, a ratio of insulating resin layer (12) thickness/film (11) thickness is preferably 1.1 to 8, more preferably 1.2 to 6. As used in this specification and claims that follows, the phrase “insulating resin layer thickness” means a thickness of the insulating resin layer formed on one face of the film. Concretely, the thickness T_(i) of the insulating resin layer 12 is for example 3 to 80 μm, and the thickness T_(f) of the film 11 is 2 to 16 μm. In this way, due to a thin film 11 of the sheet member 10, the interlaminar junctions are easy to form. In addition, because the wiring patterns 20 (20 a,20 b) are buried in the insulating resin layers, the spacing between the front-sided wiring pattern 20 a and the rear-sided wiring pattern 20 b is small. Thus, the front-sided wiring pattern 20 a and the rear-sided wiring pattern 20 b are easy to joint to each other upon being pressed. For example, the spacing between the front-sided wiring pattern 20 a and the rear-sided wiring pattern 20 b is preferably 2 to 15 μm, more preferably 2 to 9 μm. Furthermore, in the case where the insulating resin layer 12 is thicker than the film 11, a sliding flexibility or a flexing life of the obtained flexible substrate is improved. The reason for this is that, when the flexible substrate is folded, the stress applied on the film and the buried wiring patterns is alleviated by the insulating resin layer having a low modulus of elasticity.

The film 11 is generally a resin film having an insulating characteristic, preferably a heat-resisting film. For example, the film 11 is a resin film made of an aramid or a polyimide. It is further preferred that an aramid film is used as the film 11. The reason for this is that the aramid film is better in terms of a surface flatness, a low absorptivity and a dimensional stability. Further reason for this is that, even in the case where the aramid film is thinner than a polyimide film, a given strength is easy to achieve, and that the aramid film is cheaper than the polyimide film. In addition to that, the aramid has a high elasticity-strength so that it is suitable for forming a thin film. For example, about 4 μm of the aramid film thickness corresponds to about 12.5 μm of the polyimide film thickness.

The insulating resin layers 12 formed on both surfaces of the film 11 serve to house the wiring patterns 20. In order to improve an adhesion strength between the insulating resin layers 12 and the wiring patterns 20, or to improve an adhesion strength between the multi layered substrates, it is preferred that the insulating resin layers 12 have an adhesive property. Therefore, the insulating resin layer is preferably made of at least one resin material selected from the group consisting of an epoxy resin, a polyimide resin, and an acrylic resin and a modified resin thereof.

The sheet member 10 used in the production process of the present invention is characterized in that the wiring patterns 20 are embedded in the insulating resin layers 12. For the purpose of obtaining these wiring patterns 20, first, a substrate 80 in which insulating resin layers 12 are formed on the front surface and rear surfaces of a film 11 is prepared as shown in FIG. 6(a). Subsequently, wiring patterns 20 a,20 b are embedded into the insulating resin layers 12 as shown in FIG. 6(b). These are preferably carried out by means of a transferring technique as shown in FIG. 7. As for such transferring technique, it is required to prepare carrier sheets 32 on which the wiring patterns 20 a,20 b are preliminarily formed. And also it is required to prepare a film 11 on both surfaces of which insulating resin layers 12 are formed. Subsequently, the carrier sheets 32 and the film 11 are conveyed in such a manner that they pass through a pair of roll members 31 rotating in a certain definite direction. As a result, they are pressed due to a nip pressure of a pair of roll members 31. This causes the wiring patterns 20 a, 20 b formed on the carrier sheet 32 to be embedded into the insulating resin layers 12 formed on the film 11. As a result of that, by removing the carrier sheet 32 (including no wiring pattern), a sheet substrate 10 in which the wiring patterns 20 a, 20 b are buried in the insulating resin layers 12 is finally obtained. In the case of the insulating resin layers 12 made of the thermosetting resin, the insulating resin layers 12 are preferably kept in a semi-curing state during an embedding process. Preferably, the wiring patterns 20 a,20 are buried in the insulating resin layers 12 in such a manner that the surfaces of the wiring patterns 20 a,20 b are on the same level or approximately the same level as both surfaces of the insulating resin layers 12. That is to say, the surface of each of the wiring pattern 20 a, 20 b is flush with the surface of each of the insulating resin layers 12 formed on the front face and the rear face of the film 11. This results in a better flatness of the sheet member 10 and the obtained flexible substrate 100, which in turn leads to an advantage for a multilayering process of the substrates. Such transferring process can give a more fine-pitch wiring pattern than a wet etching process. For example, a line/space (L/S) of the wiring patterns for the case of the wet etching process is 40 μm/40 μm, whereas a line/space (L/S) of the wiring patterns for the case of the transferring process is very fine 15 μm/15 μm (i.e. 30 μm pitch).

The wiring patterns 20 may be made of any materials if they have an electrically conductive properties. For example, it is preferred that the wiring patterns 20 are made of metal materials selected from the group consisting of a copper, a nickel, a gold and a silver. It is also preferred that the carrier sheet used in the transferring technique is made of organic films such as a PET, or an metal foil such as a copper foil, and that it is therefore something like a sheet-like member which is about 25 to 200 μm in thickness.

Next, the step (b) will be hereinafter described. In the step (b), a part of at least one of the front-sided and rear-sided wiring patterns is pressed into the inside of the sheet member in order to crash through the insulating resin layer(s) and film, and thereby a part of the front-sided wiring pattern and a part of the rear-sided wiring pattern are jointed to each other. This causes a part of the front-sided wiring pattern and a part of the rear-sided wiring pattern to be pressure-jointed to each other. Therefore, the step (b) is herein referred to also as “pressure-joint step”. However, it should be noted that if the front-sided wiring pattern and the rear-sided pattern are electrically connected to each other, it is no longer required to be pressed more, and therefore the term “joint” does not necessarily mean the phrase “pressure joint” herein. It is preferred that a pressing direction is approximately perpendicular to a plane of the sheet member. In FIGS. 4 and 5, the junction (i.e. interlaminar junction) formed due to the above “joint” is indicated by number 25.

As a pressing tool, a needle-like member, or a roll member 33 with protrusions 35 (see FIG. 8) may be used. In the case where the roll member is used, it is preferably used as a pair of roll members in order to utilize a nip pressure thereof. FIG. 9 shows an embodiment wherein the flexible substrate 100 is produced by means of the needle-like member 50. It is preferred that a pressing portion (i.e. tip) of the needle-like member 50 is hemispherical in shape. However, a planar tip of the needle-like member 50 is permitted. Likewise, it is preferred that the pressing portions of protrusions 35 of the roll member 33 are hemispherical in shape. However, planar tips of the protrusions 35 are also permitted. The needle-like member and the roll member are preferably made of a stainless material (i.e. SUS), a nickel or an aluminum and so on.

In the case where both of a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern are pressed, a cross-sectional view of a wiring section composed of those portions is approximately “X” in shape. In contrast, in the case where one of a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern is pressed, a cross-sectional view of a wiring section composed of the pressed portion is approximately “U” in shape.

In the case where both of a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern are pressed, it could lead to an embodiment wherein a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern are jointed to each other within the interior of the film. In some cases, it could lead to an embodiment wherein a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern are jointed to each other within the interior of the insulating resin layer in such a manner that one of them penetrates through the film. In the meanwhile, in the case where one of a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern is pressed, it could lead to an embodiment wherein a portion of the front-sided wiring pattern and a portion of the rear-sided wiring pattern are jointed to each other in such a manner that the pressed portion of the wiring pattern only penetrates through the film.

A pressing operation of the step (b) can be carried out at normal temperature (e.g. 20 to 80° C.). A pressing force applied for each interlaminar junction is preferably 100 to 1200 gf, more preferably 500 to 1000 gf.

In the step (b), in the case where the pressing operation is carried out by means of the roll member with protrusions while the sheet member is moving unidirectionally, it is possible to form the interlaminar junctions continuously. Thus, it is possible to employ a roll-to-roll process in the production process (I), which in turn leads to a mass production of the flexible substrate.

In a preferred embodiment, as shown in FIG. 10, conductive member 27 may be disposed on at least one of the front-sided and the rear-sided wiring patterns 20 in the step (b) in such a manner that the disposed conductive member 27 projects from the surface of the wiring pattern. Thus, the conductive member is herein referred to also as “conductive projecting member”. Pressing the conductive projecting member 27 causes at least one of a part 22 a of the front-sided wiring pattern 20 a and a part 22 b of the rear-sided wiring pattern 20 b to be indirectly pressed into the sheet member 10. Incidentally, as to the embodiment as shown in FIG. 10, the conductive projecting members 27 are disposed on both of the front-sided wiring pattern 20 a and the rear-sided wiring pattern 20 b. That is, both of the front-sided wiring pattern 20 a and the rear-sided wiring pattern 20 b are pressed into the sheet member 10 through the intermediary of the conductive projecting members 27.

In such an embodiment that the conductive projecting members 27 are disposed, the concave portions (i.e. depressed portions) in the surface of the wiring pattern, which portions are formed due to the pressing, are supposed to be filled with the conductive members 27 so that the surface of each wiring pattern is flat, as shown in FIGS. 11(a) or 11(b). As a result, the surface of the flexible substrate 100 becomes flat, which in turn leads to a construction suitable for a multilayering process. In such flexible substrate 100, due to the filled conductive member 27, the resistance of each interlaminar junction becomes low, which in turn allows a large electric current to pass therethrough if necessary.

In the case where a part of the front-sided wiring pattern or a part of the rear-sided wiring pattern is pressed through the intermediary of the conductive projecting member, a needle-like member or a roll member may be used. FIG. 12 shows an embodiment wherein a part 22 a of the front-sided wiring pattern 20 a and a part 22 b of the rear-sided wiring pattern 20 b are pressed by means of the needle-like members 50 through the intermediary of the conductive projecting members 27. In the case where the roll member 34 is used, it is preferred that the roll member 34 is a cylindrical in shape. In this case, it is also preferred that the roll member 34 is made of materials such as styrol material or rubber.

It is preferred that the conductive projecting member 27 is mainly made of a metal, for example an alloy. It is preferred that such alloy mainly consists of a metal selected from the group consisting of a copper, a nickel, an aluminum, a gold, a silver and a combination thereof. Also, as a material of the conductive projecting member 27, a conductive paste which contains carbon powder or the above-mentioned metal in powder may be used.

In order to form the conductive projecting member 27, a paste printing technique, a bump forming technique, a ball mounting technique, or an electrophotography technique may be employed. That is, in the case of employing the paste printing technique, a projecting conductive member is formed by paste-printing a conductive material and thereafter drying it. In the case of employing the bump forming technique or the ball mounting technique, the conductive projecting members are formed by forming bumps on a metal layer, or mounting metal balls on a metal layer. As to the electrophotography technique, an embodiment thereof is shown in FIG. 14. In such electrophotography technique, subsequently to electrically charging a photoconductive drum 51 by means of a electrification device 52 (i.e. electrification roll), an electrostatic latent image is formed at a predetermined position of the photoconductive drum surface by means of an optical writing technique using a light source 53 (e.g. LED or laser). Subsequently, the conductive members 27 are attached to the photoconductive drum 51, and thereafter the conductive members 27 are transferred to the surface of a wiring pattern 20. As a result, the conductive projecting members 27 are formed on the surface of a sheet member.

In the case where the conductive projecting member is disposed on one of the front-sided wiring pattern and the rear-sided wiring pattern, not on both of them, the cross-sectional view of a portion of the wiring section composed of the pressed part of one of the front-sided wiring pattern and the rear-sided wiring pattern becomes approximately “U” in shape after the pressing step (b) is carried out with the roll member 34. Incidentally, not before the pressing step (b) but after that, the conductive projecting member may be formed. That is, in order to achieve the flat surface of the wiring pattern as well as a low resistance of the interlaminar junction, the concave portion on the surface of the wiring pattern, which portion is formed due to the pressing step (b), may be filled with the conductive material.

In a preferred embodiment of the step (b), a part of the front-sided wiring pattern and a part of the rear-sided wiring pattern may be jointed to each other through the intermediary of the solder. For example, as shown in FIG. 15, a sheet member 92 in which the solders 15 are disposed between the film 11 and the wiring pattern 20 is pressed by means of the roll member 33, so that the obtained interlaminar junctions contain the solder 15. It is preferred that the obtained sheet member 90 having interlaminar junctions is subjected to a reflow process using an oven 70. In the case where the solder is used in producing a flexible substrate 100, the solder are supposed to be additionally included in the junction composed of a part 22 a of the front-sided wiring pattern 20 a and a part 22 b of the rear-sided wiring pattern, which will lead to a preferable reliability in connection.

As shown in FIG. 16, the solder 15 may be used even in the case where the interlaminar junctions are formed through the intermediary of the conductive projecting members 27. In this case, such sheet member 92 that the solders 15 are disposed between the film 11 and the wiring patterns 20 is used, and the conductive projecting members 27 are disposed on the sheet member 92. The obtained flexible substrate 100 is better in terms of the flatness and the connecting reliability.

In a preferred embodiment, a heat treatment of the joint-surface of the junction may be carried out so as to improve the connecting condition of the junction. For example, as shown in FIG. 17, the heat treatment of the joint-surface 22 c of the junction 25 can be successively carried out by moving a welding appliance up and down (i.e. direction indicated by the arrow 62 and 64) relative to a horizontal moving direction (i.e. direction indicated by the arrow 40) of the flexible substrate 100. In stead of the heat treatment (i.e. a welding treatment), a laser machining treatment or an electric discharge machining may be employed.

In particular, as shown in FIG. 18, it is preferred that an ultrasonic wave is applied to the junction 25. In the embodiment shown in FIG. 18, an ultrasonic applying tools 60 are reciprocated in the direction (i.e. direction indicated by the arrow 62 and 64) that is approximately orthogonal to the moving direction of the flexible substrate 100. This will cause the ultrasonic applying tools 60 and the flexible substrate 100 to contact and non-contact repeatedly, which in turn leads to a repeated on-off of the ultrasonic wave application. The application of the ultrasonic wave causes the interlaminar junction (i.e. junction 25) to be ultrasonically bonded. As a result, the strength of the interlaminar junction as well as the connecting reliability is improved. Furthermore, in this case, the resistance of the interlaminar junction 25 decreases because the region adjacent the contact-surface 22 c (i.e. boundary face) is melted due to the ultrasonic bonding. For example, the resistance of the interlaminar junction after the application of the ultrasonic wave is less than half resistance before the application thereof. This enables a large electric current to pass through the junction, and also leads to a decrease of an electric power consumption. It is preferred that the frequency of the ultrasonic vibration is about 15 to 150 kHz, and that the generating power is about 10 to a few thousand W. Also, it is preferred that the applying time is 0.1 to 10 second (typically about 1 second), which corresponds to the applying energy of 1 to a few KJ.

Not only an embodiment wherein the ultrasonic wave is applied after the formation of the interlaminar junctions is possible, but also an embodiment wherein the ultrasonic wave is applied during the formation of the interlaminar junctions is possible. In the case where the ultrasonic wave is applied during the formation of the interlaminar junctions, it is preferred that the needle-like member or the roll member having protrusions is used. In other words, the pressing step (b) is carried out by means of the needle-like member or roll member, both of which are respectively provided with a function of applying an ultrasonic wave. For example, as to the needle-like member, it is preferred that the tip of the needle-like member is provided with such function.

In a further embodiment, it is preferred that the interlaminar junctions are heated before or after being treated by an ultrasonic wave. This will cause the region adjacent the interlaminar junctions to soften and thereby such region becomes easy to transform. As a result, a desirable ultrasonic bonding can be obtained due to a lager connecting area. The sheet member or the flexible substrate may be placed on the heated roll member or conveyor because a heating of the sheet member itself results in a heating of the interlaminar junctions. In this case, the sheet member or flexible substrate may be heated to for example 50 to 400° C., preferably 100 to 300° C.

It is preferred that the application of the ultrasonic wave is carried out while measuring a physical characteristic of a part of the wiring patterns. For example, the measured physical characteristic is a resistance of a part of each wiring pattern, or a degree of the dent or depression of each wiring pattern. In the case where the application of the ultrasonic wave as well as the measurement of the physical characteristic (e.g. resistance) are concurrently carried out, a strength characteristic of the interlaminar junction can be obtained in real time, which in turn allows the ultrasonic wave to be applied to such a degree that a desired strength characteristic is obtained. It is only necessary to carry out the measurement of the physical characteristic at first one time or a few times. That is to say, according to the result of such measurement, it is afterward possible to adjust the amount of the energy of the ultrasonic wave (e.g. an applying time or an amount of the ultrasonic wave).

In the case where the ultrasonic wave is applied, the conductive members 15 may be disposed on the inner side of the wiring patterns 20 in the sheet member 94 as shown in FIG. 19. It is preferred that the conductive member 15 is made of a metal selected from the group consisting an aluminum, a gold, a silver, a platinum and a vanadium. If necessary, a solder may be contained in the conductive member 15.

According to the embodiment as shown in FIG. 19, a part of each of the front-sided and the rear-sided wiring patterns is pressed toward the interior of the sheet member when a sheet member 94 passes through the roll member 33. Therefore, the front-sided wiring pattern 20 a and the conductive members 15 inside thereof, and the rear-sided wiring pattern 20 b and the conductive members 15 inside thereof are jointed to each other. As a result of that, the interlaminar junctions 25 including conductive members 15 are obtained. Subsequently, by means of the ultrasonic applying tool 60, the ultrasonic wave is applied to the sheet member 25 having such interlaminar junctions 25 in order to ultrasonically bond the junctions 25. As for the obtained flexible substrate 100, not only an alloy consisting of the metals (e.g. copper) of the same kind, but also another alloy consisting of the metals of the various kinds is contained in the junction, so that the strength of such junction is further improved. This will lead to achievement of the flexible substrate having a better reliability in connection.

Hereinabove, the production process (I) has been described. As shown in FIGS. 4 and 5, the flexible substrate obtained by such production process (I) comprises:

-   -   a film 11,     -   insulating resin layer 12 formed on each of a front face of the         film 11 and a rear face of the film 11 which face is opposite to         the front face, and     -   a front-sided wiring pattern 20 a embedded in the insulating         resin layer 12 formed on the front face of the film 11, and a         rear-sided wiring pattern 20 b embedded in the insulating resin         layer 12 formed on the rear face of the film 11,     -   wherein a part of at least one of the front-sided wiring 20 a         and the rear-sided wiring 20 b is dented toward interior of the         flexible substrate in such a manner that it penetrates through         the insulating layer 12 and film 11, so that a part 22 a of the         front-sided wiring 20 a and a part 22 b of the rear-sided wiring         20 b are jointed to each other to form junction.

By using of this flexible substrate 100, it is possible to produce a multilayer flexible substrate 150 as shown in FIG. 20. In the multilayer flexible substrate 150 shown in FIG. 20, the flexible substrates 100 obtained by the production process (I) are stacked on both surfaces of the flat layer 110 (i.e. mat-like layer) having a metal layer 21. Also, it is possible to stack typical substrates (e.g. substrates 500 as shown in FIG. 1(f)) on the flexible substrate 100 (serving as a core substrate) obtained by the production process (I). These multilayer substrates are characterized in that they are extremely thin. For example, in the case where the flexible substrate obtained by the production process (I) is 24 μm in thickness, the four-layer lamination thereof results in less than 100 μm in thickness, and six-layer lamination thereof results in less than 150 μm in thickness. This will lead to achievement of a very thin multilayer flexible substrate.

Hereinafter, the steps for producing a flexible substrate 100 of the present invention as well as a transferring step will be described with respect to FIG. 21.

First, as shown in FIG. 21, a substrate 80 in which insulating resin layers 12 are formed on both surfaces of a film 11 is conveyed in the direction of the arrow 40. When the substrate 80 passes trough a pair of roll members (i.e. nip rolls), the wiring patterns 20 (i.e. 20 a and 20 b) are transferred to the insulating resin layers 12.

Before such transferring step, it is required that the wiring patterns 20 are disposed on carrier sheets 32. Due to a rotating of the roll members 31, the carrier sheets 32 are conveyed toward the direction indicated by the arrow 42. Thus, due to a nip pressure of the roll members 31 will cause the wiring patterns 20 (i.e. 20 a and 20 b) to be pressed toward the insulating resin layers 12 of the substrate 80, which will embed the wiring patterns 20 into the insulating resin layers 12.

After the wiring patterns 20 (i.e. 20 a and 20 b) are embedded, a sheet member 90 moving in the direction indicated by the arrow 41 passes through a pair of roll members 33. As shown in FIG. 21, this roll member 33 a is provided with a plurality of protrusions in such a manner that the protrusions correspond to a pattern of the interlaminar junction (i.e. so-called “via”) to be formed. The rotating (in the direction indicated by the arrow 44) of a pair of roll members 33 causes a part 22 a of the wiring pattern 20 a and a part 22 b of the wiring pattern 20 b to be pressed toward the interior of the sheet member 90 upon contacting. As a result, a part 22 a of the wiring pattern 20 a and a part 22 b of the wiring pattern 20 b crash through the thin film 11 and thereby they are jointed to each other to form the junction.

The embodiment wherein a part of each wiring pattern is pressed through the intermediary of the conductive projecting member is shown in FIG. 22. In the embodiment shown in FIG. 22, the conductive projecting members are disposed between a transferring step and a pressure-joint step. A sheet member 90 on which the conductive projecting members 27 are formed passes through a pair of roll members 34. A pair of roll members 34 rotates in the direction indicated by the arrow 44. This causes a part 22 a of the wiring pattern 20 a and a part 22 b of the wiring pattern 20 b to be pressed toward the interior of the sheet member 90 through the conductive projecting members 27 when the members 27 are contacted with the roll member 34. As a result, a part 22 a of the wiring pattern 20 a and a part 22 b of the wiring pattern 20 b crash through the thin film 11 and thereby they are jointed to each other. The conductive projecting members 27 which have been pressed toward the interior of the sheet member 90 by means of a pair of roll members 34 are supposed to be located in the concave portions formed in the surface of the wiring patterns. Therefore, the obtained flexible substrate 100 becomes better in terms of surface flatness.

In a preferred embodiment, as shown in FIG. 23, convex portions 26 may be formed on the wiring pattern by means of a roll member 36 with concave portions 35′. In this case, the convex portions 26 are formed by a bounce force due to a roll press, not by a press force due to the roll press. By means of the roll member 36, it is possible to form interlaminar junctions by using the obtained convex portions 26 as bumps. Also, as shown in FIG. 24, it is possible to employ a roll member 37 with protrusions 35 and concave portions 35′. In the embodiment shown in FIG. 24, by means of the roll member 37, the convex portions 26 and the interlaminar junctions are formed subsequently to a transferring step.

Turning now to FIG. 25, a roll-to-roll process will be hereinafter described. FIG. 25 shows a step in a process for producing a flexible substrate 100 by means of the roll-to-roll process. In this process, the roll processes are carried out from beginning to end. As shown in FIG. 25, prior to transferring step, a substrate 80 is kept wound around a roll 30 a. The substrate 80 is conveyed in the direction indicated by the arrow 40 upon transferring of the wiring patterns 20 and forming of the interlaminar junctions 25. After the wiring patterns 20 are transferred and the interlaminar junctions 25 are formed, the obtained flexible substrate 100 is supposed to be finally wound around a roll 30 d. Such roll-to-roll process is suitable for a mass production because a flexible substrate and a multilayer flexible substrate are easy to hold and thereby those flexible substrates can be continuously produced. The reason why the production process of the present invention can employ the roll-to-roll process is that all the processes carried out in such production process are dry processes.

The embodiment wherein a multilayer flexible substrate 150 is produced by means of the roll-to-roll process using a flexible substrate 100 is shown in FIG. 26. In such embodiment, a film member 110 is used as a core substrate wherein insulating resin layers 12 are formed on both surfaces of the flat metal layer 21 (i.e. flat copper foil) serving as a shield layer. From a roll member 30 a, the film member 110 is conveyed in the direction indicated by the arrow 40. From a roll member 30 b, the flexible substrates 100 are conveyed toward both sides of the film member 110. A pair of the roll members 38 rotates in the direction indicated by the arrow 45, so that the flexible substrate 100 and the film member 110 are pressed due to a nip pressure of the roll members 38. The obtained multilayer flexible substrate 150 is conveyed in the direction indicated by the arrow 40, and then is finally supposed to be wound around a roll 30 d. Afterward, the multilayer flexible substrate 150 may be cut into a predetermined size, or may be subjected to a further multilayering process.

In a preferred embodiment, a roll member 33 may be employed as shown in FIG. 27, so that the a transferring step of a wiring pattern 20 and a pressure-joint step can be substantially concurrently carried out. This will lead to achievement of more efficient production process. In the shown embodiment, not only a substrate 80 but also carrier sheets 32 on which wiring patterns 20 are preliminarily formed pass through a pair of roll members 33. Thus, the wiring patterns 20 located on the carrier sheets 32 are embedded into the insulating resin layers 12, and concurrently a portion (22 a, 22 b) of each wiring pattern (20 a, 20 b) is pressed to form an interlaminar junction.

Hereinabove, the production process (I) of the present invention and the flexible substrate obtained thereby have been described. In the second place, the production process (II) of the present invention and the flexible substrate obtained thereby will be hereinafter described. The production process (II) comprises the steps of;

-   -   (a₁) preparing the sheet member comprising,         -   (i) a film,         -   (ii) an insulating resin layer formed on each of a front             face of said film and a rear face of said film which face is             opposite to said front face, and         -   (iii) a wiring pattern embedded in the insulating resin             layer formed on the front face of said film     -   (a₂) preparing the substrate having a wiring pattern formed on a         front face thereof,     -   (b) stacking said sheet member on said substrate with adjustment         of the position in such a manner that the insulating resin layer         formed on said rear face of said film of said sheet member is         contacted with said front face of said substrate, and thereafter         a part of the wiring pattern of said sheet member is pressed         toward said substrate so that said part of the wiring pattern of         said sheet member and a part of the wiring pattern of said         substrate are jointed to each other to form a junction.

FIG. 28 shows a cross-sectional view of a construction of a flexible substrate 200 obtained by the production process (II) of the present invention. The shown flexible substrate 200 comprises a sheet member 210 and a substrate 215 having a wiring pattern 17 formed on a front face thereof,

-   -   wherein the sheet member 210 comprising         -   a film 11,         -   an insulating resin layer 12 formed on each of a front face             of the film 11 and a rear face of the film 11 which face is             opposite to the front face, and         -   a wiring pattern 20 embedded in the insulating resin layer             12 formed on the front face of the film 11,     -   the sheet member 210 is stacked on the substrate 215 in such a         manner that the insulating resin layer 12 formed on the rear         face of the film 11 of the sheet member 210 is contacted with         the front face of the substrate 215,     -   a part 22 of the wiring pattern 20 of the sheet member 210 is         dented toward the substrate 215 so that a part 22 of the wiring         pattern 20 of the sheet member 210 and a part 17 a of the wiring         pattern 17 of the substrate 215 are jointed to each other to         form junction.

In the embodiment shown in FIG. 28, the insulating resin layer 12 is formed not only on the front face of the film 11 but also on the rear face thereof. The front-sided insulating resin layer 12 serves to house the wiring patterns 20, whereas the rear-sided insulating resin layer 12 serves to improve an adhesive bonding between the sheet member 210 and the substrate 215. Therefore, only in terms of housing the wiring patterns 20, it is only necessary to form the insulating resin layer 12 only on one face of the film 11. However, in terms of an adhesive bonding between the sheet member 210 and the substrate 215, it is additionally required to form the insulating resin layers 12 on both faces of the film 11, as shown in FIG. 28.

As to the flexible substrate 200 obtained by the production process (II), a part 22 of the wiring pattern 20 of the sheet member 210 and a part 17 a of the wiring pattern 17 of the substrate 215 are jointed to each other with pressure, as shown in FIG. 28. The junctions 25 formed due to the joint serve as so-called “interlaminar junctions”.

It should be noted that the sheet member 210 prepared in the production process (II) is different from the sheet member 10 prepared in the production process (I) in that the wiring pattern is formed only on one face of the film in the sheet member 210 of the production process (II) whereas the two wiring patterns are formed on both faces of the film in the sheet member 10 of the production process (I). Incidentally, the sheet member 210 may be referred to also as “flexible wiring layer” because it could be understood that the sheet member 210 is flexible layer having the wiring patterns.

As for the sheet member 210, it is preferred that the film 11 is thinner than the insulating resin layer 12. For example, a ratio of insulating resin layer (12) thickness/film (11) thickness is preferably 1.1 to 8, more preferably 1.2 to 6. As with the production process (I), the phrase “insulating resin layer thickness” means a thickness of an insulating resin layer formed on one face of the film 11. Concretely, the thickness T_(i) of the insulating resin layer 12 is 3 to 80 μm, and the thickness T_(f) of the thickness of the film 11 is 2 to 16 μm, for example. In this case, due to a thin film 11, the interlaminar junctions are easy to form therethrough. In addition, because the wiring pattern 20 is buried in the insulating resin layer 12, the spacing between the wiring pattern 20 of the sheet member 210 and the wiring pattern 17 of the substrate 215 is small. Thus, the wiring pattern 20 of the sheet member 210 and the wiring pattern 17 of the substrate 215 are easy to joint to each other upon being pressed. For example, the spacing between the wiring pattern 20 of the sheet member 210 and the wiring pattern 17 of the substrate 215 is preferably 2 to 15 μm, more preferably 2 to 9 μm. As with the production process (I), in the case where the wiring pattern is embedded in the insulating resin layer 12 that is thicker than the film 11, a sliding flexibility or a flexing life of the obtained flexible substrate 200 is improved.

The film 11 is generally a resin film having an insulating characteristic, preferably a heat-resisting film. For example, the film 11 is a resin film made of an aramid or a polyimide. As with the production process (I), it is preferred that an aramid film is used as the film 11.

The insulating resin layer 12 serves to house the wiring pattern 20 as well as enhance an adhesion strength between the multilayered substrates. Thus, it is preferred that the insulating resin layer has an adhesive property. Therefore, it is preferred that the insulating resin layer is made of at least one resin material selected from the group consisting of an epoxy resin, a polyimide resin, and an acrylic resin and a modified resin thereof, for example.

As to the sheet member 210 used in the production process (II) of the present invention, the wiring pattern is buried in the front-sided insulating resin layer 12. As with the production process (I), it is preferred that the transferring technique are employed to obtain the wiring pattern 20. Also, it is preferred that the wiring pattern 20 is made of the material as described with respect to the production process (I).

The “substrate 215 having a wiring pattern 17 formed on a front face thereof” prepared in the step (a₂) has a flexibility. The substrate 215 is not limited if it has a flexibility. Thus, a typical flexible substrate 500 as shown in FIG. (f) may be used as the substrate 215. Although only on one face of the substrate 215 shown in FIG. 28 the wiring pattern 17 is formed, another wiring pattern 17 may be additionally formed on the other face thereof. In the case where a multilayer flexible substrate is produced, various types of flexible substrates are stacked on the substrate 215. That is to say, the substrate 215 serves as a base substrate (i.e. core substrate).

Next, the step (b) of the production process (II) will be hereinafter described. In the step (b), a part 22 of the wiring pattern 20 of the sheet member 210 is pressed toward the substrate 215 so that such part 22 penetrates through the insulating resin layers 12 and film 11. As a result, a part 22 of the wiring pattern 20 of the sheet member 210 and a part 17 a of the wiring pattern 17 of the substrate 215 are jointed to each other. Prior to a pressing step, the sheet member 210 is stacked on the substrate 215 in such a manner that the rear-sided insulating resin layer 12 of the sheet member 210 is contacted with a front face of the substrate 215. In this case, the adjustment of the position is carried out in such a manner that the wiring pattern 20 of the sheet 210 is opposed to the wiring pattern 17 of the substrate 215.

As with the production process (I), a needle-like member or a roll member 33 with protrusions 35 (see FIG. 8) may be used as a pressing tool.

After a portion of the wiring pattern is pressed in the step (b), the cross-sectional view of the pressed portion is approximately “U” in shape as shown in FIG. 28. As shown in FIG. 29, the concave portions in the surface of the wiring pattern, which portions are formed due to the pressing, may be filled with a conductive material 27 in order to flatten the surface of the wiring pattern. It is preferred that the conductive material 27 consists of the material as described with respect to the production process (I).

Although the sheet member 210 is disposed only on one face of the substrate 215 in the examples shown in FIGS. 28 and 29, two sheet members 210 may be disposed on both faces of the substrate 215. Also, additional another sheet member 210 may be disposed on such sheet member 210. Because the flexible substrate 200 obtained by the production process (II) is better in terms of the surface flatness, there is little possibility of a displacement of the wiring pattern. This will lead to an advantageous construction suitable for multilayering process. As with the production process (I), the multilayer flexible substrate obtained by using of the flexible substrate 200 is extremely thin.

FIG. 30 shows a perspective view of an example of a multilayer flexible substrate 205 in which the flexible substrate 200 obtained by the production process (II) is used. In the multilayer flexible substrate 205, the sheet members 210 are formed on both surfaces of the substrate 215. The interlaminar junctions 25 are composed of a part 17 a of the wiring pattern 17 formed on the surface of the substrate 215 and a part 22 of the wiring pattern 20 of the sheet member 210. In the shown embodiment, the concave portions in the surface of the wiring pattern, which portions are formed due to the pressing or dent, are filled with a conductive material (i.e. conductive member). However, as a modified embodiment of the multilayer flexible substrate 205, no filling of the conductive material is also possible.

As shown in FIG. 30, by using of the interlaminar junctions 25, a closed region or approximately closed region 29 can be formed in the multilayer flexible substrate 205. As for a conventional method for making the via holes, such closed region 29 is extremely hard to make because there is a possibility of falling of the center region. Thus, the structure of the closed region or approximately closed region 29 is peculiar to the multilayer flexible substrate 205 of the present invention. For example, the wiring patterns 20, which is connected to the interlaminar junctions that constitute the closed region or approximately closed region 29, may be used as a gland, whereas another interlaminar junctions formed within the region 29 may be used for signal. This will allow the flexible substrate 205 to have a high tolerance to the noise.

Next, the production process (II) of the present invention will be hereinafter described with respect to the FIG. 31. FIG. 31 schematically shows a process for producing a multilayer flexible substrate 220 including a flexible substrate 200 of the present invention.

First, a sheet member 210 in which wiring pattern is buried in an insulating resin layer 12 formed on a film 11 is prepared, and a substrate 215 having a flexibility is also prepared. As to the substrate 215, wiring patterns 17 are formed on both surfaces thereof, and the front-sided wiring pattern 17 and the rear-sided wiring pattern 17 are electrically connected to each other through a via 18 (e.g. conductive paste portion). In the shown embodiment, a thermosetting adhesive layer 12′ is disposed on an organic film 11′ made of a polyimide, and the wiring patterns 17 made of a copper are formed on the thermosetting adhesive layer 12′. From a roll 30 a, the substrate 215 is conveyed in the direction indicated by the arrow 40. In contrast, the sheet member 210 superposed on a carrier sheet 32 is conveyed from a roll 30 b. As shown in FIG. 32, the sheet member 210 may be preliminarily prepared by means of the transferring technique similar to that of the production process (I).

As shown in FIG. 31, the substrate 215 and the sheet member 210 pass through a pair of roll members 38 (i.e. nip roll), so that the sheet member 210 is superposed on the substrate 215. During that, the wiring pattern 17 of the substrate 215 is embedded into the insulating resin layer 12 of the sheet member 210. The carrier sheet 32 serving to carry the sheet member 210 is supposed to be finally wound around a roll 30 c.

Subsequently, the conductive members 27 are disposed on the obtained stacked substrate 90 in such a manner that they project from the surface of the substrate 90. That is to say, the substrate 90 is provided with the conductive projecting members 27. Incidentally, the conductive projecting members 27 are formed on the wiring pattern 20 in such a position that they align with the pattern of the interlaminar junctions (so-called “via”). As with the production process (I), the conductive projecting member 27 may be formed by means of a paste printing technique, a bump forming technique, a ball mounting technique or an electrophotography technique.

Subsequently, the stacked substrate 90 on which the conductive projecting members 27 are formed passes through a pair of roll members 34. This causes the conductive projecting members 27 to be pressed into the inside of the stacked substrate 90. As a result of that, the interlaminar junctions are formed, and conductive members 27 is supposed to be located in the concave portions (formed due to the pressing) in the surface of the wiring pattern. Concretely, when the roll members 34 are contacted with the conductive projecting member 27, the conductive projecting member 27 is pressed toward the interior of the stacked substrate 90, and consequently a part 22 of the wiring pattern 20 of the sheet member 210 is pressed toward the interior of the stacked substrate 90. As a result, a part 22 of the wiring pattern 20 and a part 17 a of the wiring pattern 17 of the substrate 215 are jointed to each other. Incidentally, due to the conductive projecting member 27 located in the concave portion in the surface of the wiring pattern, a surface flatness of the multilayer flexible substrate 220 can be achieved. Subsequently, the obtained multilayer flexible substrate 220 having the interlaminar junctions is conveyed in the direction indicated by the arrow 40, and then is supposed to be wound around a roll 30 d, and finally cut into a predetermined size. In the case where further multilayering process is carried out, the obtained multilayer flexible substrate 220 may be subjected to a further superposing step.

As for the embodiment shown in FIG. 31, the sheet member 210 is superposed on the one side of the substrate 215. However, as shown in FIG. 33, two sheet members 210 may be superposed on the both sides of the substrate 215.

Also, as for the embodiment shown in FIG. 31, the pressure-joint step is carried out through the intermediary of the conductive projecting members as well as by means of the roll members 34 with no protrusion. However, as shown in FIG. 34, not through the intermediary of the conductive projecting members, but only by means of the roll members 33 with protrusions 35, the pressure-joint step may be carried out.

Furthermore, as shown in FIG. 35(a), convex portions 26 may be formed by means of a roll member 37 with concave portions 35′ as well as protrusions 35, and subsequently by using the obtained convex portions 26 as bumps, interlaminar junctions 25 a may be formed as shown in FIG. 35(b).

In the production process (II), the flexible substrate 100 as obtained by the production process (I) may be used as the substrate 215 having a flexibility. The embodiment of this case is shown in FIG. 36. As for the construction of the substrate 215 shown in FIG. 36, the wiring patterns are buried in insulating resin layers 12′ formed on both surfaces of a film 11′, and a part of the front-sided wiring pattern 20 a and a part of the rear-sided wiring pattern 20 b constitute interlaminar junction 25 b. Additionally, in the embodiment shown in FIG. 36, the concave portions formed due to the interlaminar junctions 25 b are filled with the conducive material. Such substrate 215 will enhance the merits of the production processes (I) and (II) because no through hole is required to be made in the sheet member 210 as well as in the substrate 215. FIG. 37 shows a structure of a multilayer flexible substrate 260 obtained by the production process shown in FIG. 36. The multilayer flexible substrate 260 shown in FIG. 37 corresponds to a modified flexible substrate 205 shown in FIG. 30 wherein the construction of the substrate 215 is changed. In the embodiment shown in FIG. 37, the wiring pattern located at a right side of the region where a conductive material 27 is filled may be used for a glad (20G), whreas the wiring pattern located at a left side thereof may be used for signal (20S).

By suitably using of the flexible substrate obtained by the production process (II), it is possible to construct a three-dimensional coil (i.e. inductor) as shown in FIGS. 38 and 39. FIG. 38 shows a cross-sectional view of the structure of the multilayer flexible substrate 270. FIG. 39 shows a perspective view of a multilayer flexible substrate 270 wherein only wiring patterns 17, 20 are seen through. The cross-section along the line A-A′ in FIG. 39 corresponds to the cross-section of the multilayer flexible substrate 270 in FIG. 38. As for the shown embodiment, a stereoscopic coil (inductor) 155, not a planar coil is composed of the wiring patterns 17,20 and the interlaminar junctions 19, 25 within a thin multilayer flexible substrate 270. This coil 155 may be used as a inductor of an extremely thin device. Also, according to the embodiment as shown in FIGS. 38 and 39, an inductor with a low direct-current resistance can be obtained because the interlaminar junctions consist of the same material as the wiring (i.e. wiring pattern). Furthermore, the three-dimensional coil 155 has an advantage in that a higher inductance value than that of the two-dimensional coil can be achieved. The reason for this is that, as for the three-dimensional coil 155, a large number of the coil turn can be easy to achieve even when the available planar region of the surface is small.

According to the production process (II), as with the production process (I), it is possible to form the interlaminar junctions easily without making the through holes and filling the conductive paste. This will lead to achievement of a simple production process of the flexible substrate. Also, as with the production process (I), a so-called roll process (i.e. roll-to-roll process) can be employed in the production process (II) because all the processes carried out in the production process (II) are dry processes. Therefore, it is possible to produce the multilayer flexible substrates continuously, which in turn leads to achievement of a mass production.

Also, as with the production process (I), the aramid film, which has a higher elastic strength than the polyimide film and therefore is suitable for forming a thin film, may be used in the production process (II). This will lead to achievement of an extremely thin flexible substrate.

Furthermore, as with the production process (I), the transferring technique may be employed in the production process (II). Therefore, it is possible to embed the wiring pattern 20 into the insulating resin layer 12 in such a manner that the surface of the wiring pattern 20 is on the same level (or approximately the same) level as a surface of the sheet member 210. That is to say, it is preferred that the surface of the wiring pattern 20 is flush with the surface of the sheet member 210. This will lead to achievement of a better flatness of the flexible substrate 200, and thereby the superposing process can be carried out easily.

Hereinabove, the production processes (I) and (II) as well as the flexible substrates 100 and 200 obtained thereby have been described.

In the next place, a process for producing a multilayer flexible substrate will be hereinafter described. Such process is characterized in that the multilayer flexible substrate is produced by using of the flexible substrates 100 and 200 obtained by the production processes (I) and (II) of the present invention.

FIG. 40 shows an example of a multilayer flexible substrate 180 in which six flexible substrates 120 are stacked to each other. At least one of six flexible substrates 120 is the flexible substrates 100 or 200 as obtained by the production processes (I) or (II).

In order to achieve a thin multilayer flexible substrate 180 (i.e. small thickness T_(i)), it is preferred that the flexible substrates 100, 200 that are respectively 10 to 25 μm in thickness. In stead of using the flexible substrates 100,200, the typical flexible substrate (e.g. flexible substrate shown in FIG. 1(f)) or flexible substrate 130 having a flat metal layer 28 (see FIG. 41) may be used, for example. It is more preferred that more than and equal to half number of the flexible substrates 120 are the flexible substrates 100, 200 obtained by the production processes (I) or (II). Incidentally, as to the adjacent two flexible substrates 120 that constitute such multilayer flexible substrate 180, the insulating resin layer of one of the flexible substrates 120 may serve as the insulating resin layer of the other flexible substrate 120. That is to say, it is a possible embodiment wherein there is only one insulating resin layer between respective films of the adjacent flexible substrates 120.

As shown in FIG. 40, electronic components 81 (e.g. semiconductor chip) can be disposed on the multilayer flexible substrate 180, which in turn leads to achievement of the flexible device 300. In the shown embodiment, the semiconductor chip 81 is connected to the wiring pattern (not shown) through the connecting members 82 (e.g. bump or solder). Also, in this embodiment, an underfill 83 is formed around the connecting portion of the semiconductor chip 81.

In the case where the thickness T₂ of the semiconductor chip 81 is for example 50 to 130 μm, and the thickness T₁ of the six-layer flexible substrate (i.e. multilayer flexible substrate 180) is for example 75 to 150 μm, the flexible device 300 is as small as 125 to 280 μm in thickness (=T₂+T₁). Such extremely thin flexible device 300 (i.e. less than and equal to 300 μm in thickness) serves many uses.

As shown in FIG. 42, it is possible to flatten the surface of the flexible device 300 where the semiconductor chip 81 is supposed to be mounted on. In the embodiment shown in FIG. 42, a composite sheet 84 made of inorganic filler and resin is formed in such a manner that the upper surface thereof is on the same level as the upper surface of the semiconductor chip 81. The reason why such composite sheet 84 is employed is that an inorganic filler contained therein severs to radiate more heat, and that it is possible to prepare such composite sheet 84 in such a manner that the thermal expansion coefficient of the composite sheet 84 is the same as that of the semiconductor chip 81.

Incidentally, a flexible device 300 having the wiring patterns (not shown) formed on the surface thereof can be obtained by transferring the composite sheet 84 having the wiring patterns formed on the surface thereof to the multilayer flexible substrate 180. As to the flexible device 300, a preferable surface flatness is achieved, so that further electronic components can be mounted to the flat surface of the flexible device 300.

As shown in FIG. 43, it is possible to construct the flexible device 300 in such a manner that the electronic components (e.g. passive components 85 a, 85 b) are included within the multilayer flexible substrate 180. For example, as shown in FIGS. 44(a) and 44(b), a transferring technique may be employed in order to include the passive components 85 a, 85 b within the multilayer flexible substrate 180. Concretely, first, wiring pattern 20 as well as the passive component 85 a (e.g. sheet-like capacitor formed by means of the printing or thin-film sputtering techniques) or the passive component 85 b (e.g. sheet-like resistor formed by means of the printing or thin-film sputtering techniques) is preliminarily disposed on a carrier sheet 32 as shown in FIG. 44(a). Subsequently, as shown in FIG. 44(b), the wiring pattern 20 as well as the passive components 85 a, 85 b is transferred to the insulating resin layer 12 of the substrate 80. As a result of that, the flexible device including the passive components can be obtained. As the passive components 85 a,85 b, an inductor, a condenser, or a resistor may be used. Incidentally, even in the case where the substrate 80 is thin, the electronic components are comparatively easy to include due to the fact that the insulating resin layer 12 is thicker than the film 11.

In order to make full use of the characteristics of the flexible substrates 100,200, the multilayer 180, or flexible devices 300, it is preferred that they are mounted to a thin compact electronics device or a compact electronics device wherein a mounting area is extremely limited. For example, as shown in FIG. 45, they may be mounted in the electronics device 400 (e.g. cellular phone).

In the thin cellular phone 400 shown in FIG. 45 (preferably extremely thin, and less than and equal to 2 to 6 μm in thickness T), the flexible substrate 100 is used as a circuit board. In stead of the flexible substrate 100, flexible substrate 200, multilayer flexible substrate 180, or flexible device 300 may be used as a circuit board of the cellular phone 400.

Within the housing 499 of the cellular phone 400, a display unit 491 (e.g. LCD panel), key unit 496 (in which antenna 492, battery 493, and buttons 496 a are mounted), camera unit 497 (e.g. CCD or CMOS image sensor) are mounted. In spite of a limited mounting region within the housing 499, it is possible to effectively use the flexible substrates 100, 200 having a better flexing life. The flexible substrate 100 serving as a circuit board can be relatively easily provided with curved sections (or flexural portions) 100 a or clinched portions 100 b, which in turn leads to achievement of a high-density mounting.

Hereinabove, although the present invention has been explained as above with reference to preferred embodiments, it will be understood by those skilled in the art that the present invention is not limited to such embodiments and can be modified in various ways. For example, in order to lower the cost of the production processes (I) or (II) of the present invention, it is possible to use a commercially available laminate with metal layer in stead of the sheet member, and preferably a copper-clad laminate is used.

As an additional remark, Japanese Patent Kokai Publication No. 3-201498, Japanese Patent Kokai Publication No. 49-27866, Japanese Patent Kokai Publication No. 55-102291, Japanese Patent Kokai Publication No. 9-283881, and Japanese Patent Kokai Publication No. 52-71677 will be hereinafter described, although the inventions disclosed in those publications are fundamentally different from the present invention in terms of their technical meanings.

Japanese Patent Kokai Publication No. 3-201498 and Japanese Patent Kokai Publication No. 49-27866 disclose the technology wherein a part of the wiring pattern is crashed through the insulative layer and then connected to the metal substrate (e.g. aluminum substrate) without a screw tool for the purpose of ensuring an electrical connection between the wiring pattern and the metal substrate. As to such disclosed technology, a connecting tool is used to connect a part of the wiring pattern to the metal substrate. By means of this connecting tool, the relatively soft surface of the aluminum substrate is deeply caved. Thus, the disclosed technology can not be employed for forming the vias of double-sided flexible substrate. Beyond that, no wiring pattern is formed in the metal plate (aluminum plate) serving as a radiator plate. Basically, what are disclosed in those publications are alternative technologies for the connection with the screw, and therefore are fundamentally different from the present invention.

Japanese Patent Kokai Publication No. 55-102291 discloses a structure of the through-hole conductor of the flexible circuit board. The technology disclosed in this publication is an alternative technology for a process for making a conventional through hole, and therefore is fundamentally different from the present invention. Further, the structure of the through-hole conductor disclosed in this publication has a possibility of less reliable connection from a structural standpoint. That is to say, as to the disclosed technology, the concern remains about the reliability in connection. The reason for this is that the circuit wiring patterns formed on one surface and the other surface are connected to each other at a so-called “shoulder position” of the through hole, and therefore a point-contact in a cross-sectional view and plane-contact in an overall view are conceivable. In contrast, as to the interlaminar junction of the present invention, a stable line-contact in a cross-sectional view and plane-contact in an overall view are conceivable. Therefore, in that respect, the technology disclosed in the publication is greatly different from the present invention. Incidentally, in the case of the plated through-hole, the disconnection of the wiring tends to take place at the “shoulder position”, and therefore it is required to take measures against such disconnection. In the meanwhile, as to the flexible substrate of the present invention, the stress occurred at the position corresponding to the above “shoulder position” is low. In such a light, they are also different from each other.

Japanese Patent Kokai Publication No. 9-283881 discloses a circuit board including pressure-welding vias. In this circuit board, the pressure-welding via is formed within the board by butting a part of the wiring disposed in the front surface of the board. During a formation of the pressure-welding via, a softening technique is carried out with heat. For example, the pressure-welding via is formed by means of a pressure-plate that is heated with a heating medium (see FIG. 2 in the publication). Therefore, the board is to be considered limited to the board consisting of a thermoplastic resin, and therefore there is a possibility of a less preferable heat-resisting characteristic of the circuit board. Japanese Patent Kokai Publication No. 52-71677 discloses a method wherein the circuit conductors formed on both faces of the print circuit board are electrically connected to each other. As to the print circuit board disclosed in the publication, the circuit conductors are formed on both surfaces of the insulating board consisting of a thermoplastic resin. Thus, such both-sided circuit conductors are pressed by heat-softening the spot to be connected, and thereby they are come close together. That is to say, they are spot-welded to each other. Therefore, as with Japanese Patent Kokai Publication No. 9-283881, the insulating board in the method disclosed in Japanese Patent Kokai Publication No. 52-71677 is considered limited to the board consisting of a thermoplastic resin, and therefore there is a possibility of a less preferable heat-resisting characteristic of the print circuit board. In contrast, the flexible substrate of the present invention has a higher heat-resisting characteristic as a whole, because the flexible substrate is composed of the thermosetting resin and the heat-resisting film. Also, as for the structure of the flexible substrate of the present invention, the wiring patterns are embedded in the insulating resin layers and thereby the spacing between the front-sided wiring pattern and the rear-sided wiring pattern is small. Furthermore, the thin film (preferably the film made of aramid suitable for forming a thin film) is used and thereby the interlaminar junctions can be formed without heat-softening. As described above, the structure of the flexible substrate of the present invention is different from those of the circuit board and print circuit board disclosed respectively in Japanese Patent Kokai Publication No. 9-283881 and Japanese Patent Kokai Publication No. 52-71677. In such a light, they are different from each other in terms of their technical meanings.

EXAMPLES

According to examples (1) to (3), the experiments about the flexible substrate and the process for producing the same were performed.

First, according to examples (1) and (2), the experiments concerning a flexing life of the flexible substrate of the present invention were performed.

Example 1

(Film Material)

A film (organic film) used in this example is shown in Table 1. TABLE 1 No. FILM NAME OF COMMODITY (MANUFACTURER) 1a ARAMID ┌MICTRON┘(TORAY Co., Ltd.) 1b ARAMID ┌ARAMICA┘(TEIJIN ADVANCED FILM Co., Ltd.) 1c PI ┌KAPTON┘(DU PONT-TORAY Co., Ltd.) 1d PI ┌UPILEX┘(UBE INDUSTRIES Co. Ltd.) 1e PEN ┌TEONEX┘(TEIJIN-DU PONT FILM Co. Ltd.) 1f PET ┌TETRON┘(TEIJIN-DU PONT FILM Co. Ltd.) 1g PPS ┌TORELINA┘(TORAY Co., Ltd) 1h PA ┌HARDEN┘(TOYOBO Co., Ltd) 1i PC ┌PANLITE┘(TEIJIN CHEMICALS Co., Ltd) 1j PES ┌SUMILITE FS-1300┘(SUMITOMO BAKELITE Co., Ltd) 1k PEI ┌SUPERIO UT┘(MITSUBISHI PLASTICS, Inc.) 1l PPE ┌DIANIUM┘(MITSUBISHI PLASTICS, Inc.) 1m PEEK ┌SUMILITE FS-1100C┘(SUMITOMO BAKELITE Co., Ltd)

(Preparation of the Substrate Used for Measuring Flexing Life)

By applying an epoxy-base thermosetting resin layer to both surfaces of the film with a roll coater process, the insulating resin layers were formed. Subsequently, the wiring patterns were embedded into the insulating resin layers.

Prior to embedding of the wiring patterns, a thin peel-apart layer (consisting of a nickel-phosphorus alloy) was prepared on both surfaces of the electrolytic copper foil having a thickness of 70 μm (which foil served as a supporting member of the wiring pattern), and then another copper foil (12 μm in thickness) was formed on the peel-apart layer by means of an electroplating technique. After that, a wiring pattern was formed by superposing a dry film resist on the formed copper foil, followed by carrying out a light exposure, a development, an etching, and a removing of the resist in series.

Subsequently, the supporting member having the wiring pattern was superposed on the insulating resin layers formed on the front face and the rear face of the film with adjustment of the position, and therefore the wiring pattern of the supporting member was embedded into the insulating resin layer by heating to a temperature of 60° C. as well as pressing at the pressure of 3 MPa for 5 minutes. Subsequently, after cooling, the only supporting member was peeled off, followed by carrying out a real curing of the insulating resin layer by heating it for an hour under the condition of 140° C. and 5 MPa. In this way, a substrate (i.e. sample substrate), which is regarded as a base member for the flexible substrate, was obtained. The specification of the obtained substrate is shown in the following Table 2. TABLE 2 MODULUS OF THICKNESS(μm) ELASTICITY (GPa) INSULATING INSULATING ORGANIC WIRING RESIN LAYER RESIN No. FILM PATTERN (ON ONE SURFACE OF FILM) FILM SUBSTRATE LAYER FILM 1a ARAMID 12 15 12 42 0.8 13 1b ARAMID 12 15 12 42 0.8 15 1c PI 12 15 12 42 0.8 3.2 1d PI 12 15 12 42 0.8 8 1e PEN 12 15 12 42 0.8 6.5 1f PET 12 15 12 42 0.8 5.5 1g PPS 12 15 12 42 0.8 3.9 1h PA 12 15 12 42 0.8 1.5 1i PC 12 15 12 42 0.8 2.3 1j PES 12 15 12 42 0.8 2 1k PEI 12 15 12 42 0.8 2.8 1l PPE 12 15 12 42 0.8 1.7 1m PEEK 12 15 12 42 0.8 3.2

(Measurement of Flexing Life)

Based on the technique of IPC-240C and JIS-C5016, the flexing life for various types of the sample substrates was measured.

Prior to the measurement, the sample substrate was fixed between two flat plates that were respectively opposed at a certain distance in such a manner that the sample substrate was folded at 180 degree so as to achieve a constant curvature. Subsequently, the two flat plates were moved to each other in parallel at a predetermined speed and stroke. That is to say, the sample substrate was repeatedly slid and moved to each other so that a reciprocating motion thereof was carried out. During that, the direct current resistance of the wiring patterns located at the curved inner surface of the sample substrate was monitored. The flexing life was regarded as a cycle number of the reciprocating motion wherein the resistance was increased by 80% compared with the initial resistance. By way of comparison, the flexing life of the copper foil used for the wiring pattern (i.e. copper foil having a thickness of 12 μm, and which was formed by electroplating technique) was also measured with a similar method to the above example.

(Result)

The result of the example is shown in FIG. 46. FIG. 46 is a graph that shows the modulus of elongation of the film versus the number of flexing (=flexing life) at a room temperature. A comparative example showed that a rupture occurred at 800 cycles of the reciprocating motion. Considering that along with reference to FIG. 46, it is understood that the substrate, which is considered as a base member for the flexible substrate of the present invention, has a preferable flexing life irrespective of the modulus of the elongation. The reason for this is that, due to the wiring patterns buried in the insulating resin layers, the applied stress tends to be dispersed by the insulating resin layer that retains the wiring pattern, which in turn prevents the development of the microcrack that may occur in the wiring pattern.

Example 2

(Preparation of the Substrate Used for Measuring Flexing Life)

In this example, by using of the method similar to the above example (1), various types of the sample substrates were prepared in such a manner that a ratio of insulating resin layer thickness/film, thickness is diversely changed. The specification of the prepared substrate is shown in the following Table 3. TABLE 3 MODULUS OF THICKNESS(μm) ELASTICITY (GPa) INSULATING INSULATING INSULATING RESIN LAYER ORGANIC WIRING RESIN LAYER (ON ONE RESIN THICKNESS/ No. FILM PATTERN SURFACE OF FILM) FILM SUBSTRATE LAYER FILM FILM THICKNESS 2a ARAMID 3 12 4 28 0.8 13 3.0 2b ARAMID 3 11 6.5 28.5 0.8 13 1.7 2c ARAMID 3 9.5 9 28 0.8 13 1.1 2d ARAMID 3 8 12 28 0.8 13 0.7 2e ARAMID 3 6 16 28 0.8 13 0.4 2f ARAMID 3 6 9 21 0.8 13 0.7 2g ARAMID 3 12 9 33 0.8 13 1.3

(Test Condition)

All of the films used in this example were aramid film (“MICTRON” manufactured by TORAY Co., Ltd.) The sample substrates 2 a to 2 e were approximately same in thickness thereof, and the sample substrates 2 c, 2 f, and 2 g were same in thickness of the film. The flexing life of such sample substrates were measured by using of the method similar to the above example (1). As another test condition, the speed (i.e. frequency) was 25 Hz, the stroke was 25 mm, and the curvature radius was 2 mm, 4 mm and 8 mm.

(Result)

The results of the example (2) are shown in the FIGS. 47 and 48. FIG. 47 is a graph that shows a curvature radius versus a flexing number (=flexing life). FIG. 48 is a graph that shows a ratio of an insulating resin layer thickness to a film thickness (=insulating resin layer thickness/film thickness) versus a flexing number (=flexing life). With reference to these graphs, it is understood that the flexible life of the sample substrate is better in the case where the insulating resin layer is thicker than the film, and that the smaller a curvature radius becomes, the more distinctive a feature concerning the flexible life becomes. The reason for this is that the stress applied on the wiring pattern and the film is supposed to be alleviated by the insulating resin layer having a low modulus of elasticity.

Example 3

In this example, the effect of the pressure-joint carried out in the production process of the present invention was confirmed. And also, the effect of the application of the ultrasonic wave to the interlaminar junctions was confirmed.

(Preparation of the Substrate Used for the Example (3))

As with the example (1), the film on both surfaces of which the insulating resin layers were formed was prepared, and also the two carrier sheets (i.e. copper foils that were respectively 70 μm in thickness) in which the wiring patterns were formed were prepared. The film used in the example (3) was aramid film (“MICTRON” manufactured by TORAY Co., Ltd.) having 4 μm in thickness. Each of insulating resin layers formed on both surfaces of the film was 10 μm in thickness. Each of the wiring patterns on the carrier sheets was formed by means of the electroplating technique in such a manner that the thickness of the wiring pattern was 9 μm. Subsequently, Each of carrier sheets on which the wiring patterns were preliminarily formed was superposed on the front-sided and rear-sided insulating resin layers of the film with adjustment of the position, and thereafter each of wiring patterns was embedded into each of the insulating resin layers by heating to a temperature of 60° C. as well as pressing at the pressure of 3 MPa for 5 minutes. After cooling, only carrier sheets were peeled off. In this way, the pre-cured sheet member in which the front-sided and rear-sided wiring patterns were formed was obtained. Here, each of the wiring patterns used was something like that providing a so-called “chain-via” in which the electrodes formed on the both surfaces of the substrate were connected and jointed to each other within the substrate. In that regard, the diameter of the electrode for forming the interlaminar junctions was 600 μm.

(The Formation of the Interlaminar Junctions)

Subsequently, by pressing the electrode formed on one surface of the pre-cured sheet member toward the interior thereof, the opposed electrodes were jointed to each other within the substrate. Consequently, the interlaminar junctions were formed in such a manner that 100 interlaminar junctions were coupled to each other. In that regard, a need-like cylindrical member (the tip thereof was hemispherical in shape) was used as a pressing tool, which member was made of stainless material and 100 μm in thickness. After pressing, the real curing was carried out by heating for an hour under the condition of a temperature of 140° C. as well as a pressure of 5 MPa. In this way, the flexible substrate in which the chain-vias (i.e. interlaminar junctions) was formed wad obtained.

(Preparation of the Flexible Substrate Treated by an Ultrasonic Wave)

By means of an ultrasonic applying tool (ULTRASONIC ENGINEERING Co., Ltd., type: USW-610Z20S), the ultrasonic vibration was applied to the interlaminar junctions (i.e. chain-via) via electrodes thereof. As a result of that, the flexible substrate treated by an ultrasonic wave was obtained. In that regard, the ultrasonic vibration was 28 kHz, and the generating power was 200 W.

(Measurement of the Resistance)

First, the resistances of the chain-vias of the above two types of flexible substrates (i.e. one is the ultrasonic-wave treated flexible substrate and the other is the ultrasonic-wave non-treated flexible substrate) were measured by means of a four-terminal measurement. Next, subsequent to measuring the resistance of the wiring, such resistance of the wiring was subtracted from the resistance of the chain-vias in order to obtain the resistance of each interlaminar junction. In that regard, the resistance of each interlaminar junction was obtained as an average value of ten samples for each flexible substrate prepared. As to the measurement of the wiring resistance, the wiring patterns were formed in such a manner that they had the same wiring length and wiring width as the wiring regions where no interlaminar junction existed, and thereafter the resistance of such wiring patterns was measured by means of the four-terminal measurement.

(Result)

FIG. 49 shows a resistance per interlaminar junction with a change of a pressing load. With the reference to FIG. 49, it was found that the resistance became close to a certain level in the case where the pressing load above a certain level was applied. Also, it was found that the application of the ultrasonic wave caused the resistance of each interlaminar junction to lower to the level less than half that before an application thereof.

(Liquid Bath Type Thermal Shock Test)

Subsequently, the liquid bath type thermal shock test was carried out as to the sample substrates prepared with a constant pressing load of 750 gf. Ten samples for each of two types substrates were employed. Regarding a 5-minute exposure of the sample substrate to two liquid baths respectively having −55° C. and 125° C. as 1 cycle, such cycles up to 2000 were carried out for each sample substrate. After the above exposure, the resistance of the interlaminar junction was measured. In that regard, the sample substrate in which a resistance change of more than and equal to 10% come out was regarded as “defective”. The result was as follows: a percent defectives for the ultrasonic-wave non-treated flexible substrate exposed at 1000 cycles was 0%; a percent defectives for the ultrasonic-wave non-treated flexible substrate exposed at 2000 cycles was 20%; a percent defectives for the ultrasonic-wave treated flexible substrate exposed at 1000 cycles was 0%; a percent defectives for the ultrasonic-wave treated flexible substrate exposed at 2000 cycles was 0%. Hereinabove, a beneficial effect of the flexible substrate of the present invention was confirmed.

(General Overview)

The following matters were derived from the example (3) (however, those matters may be altered to suit the design condition the flexible substrate):

It is preferred that the frequency of the ultrasonic vibration is approximately in the range of between 15 kHz and 150 kHz. The reason for this is that the ultrasonic vibration above such range will cause too large generating power, which in turn leads to an unfavorable condition concerning a high-precision processing, whereas the ultrasonic vibration below such range will cause too small generating power, which in turn leads to an insufficient melting.

It is preferred that the generating power is approximately in the range between 10 W and a few thousand W. The reason for this is also that the generating power above such range will lead to an unfavorable condition concerning a high-precision processing, whereas the generating power below such range will lead to an insufficient melting.

It is preferred that the applying time is in the range between 0.1 second and 10 second (typically about 1 second), which corresponds to the applying energy of 1 kJ to a few KJ.

INDUSTRIAL APPLICABILITY

The flexible substrate, multilayer flexible substrate, flexible device, all of which are obtained by the production process of the present invention, can be used as a circuit board of a cellular phone. However, not applying to the cellular phone, they can be used for a PDA or a notebook computer. Furthermore, they can be used for a digital still camera or a wall-hung thin-shaped television (i.e. flat-panel display). As the use of the flexible substrate in more various fields progresses, it is conceivable that a technical value of the flexible substrate (in particular the multilayer flexible substrate) of the present invention will increase more than ever.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present application claims the right of priority of Japanese Application No. 2004-079848 (filed Mar. 19, 2004, the title of the invention: “FLEXIBLE SUBSTRATE, MULTILAYER FLEXIBLE SUBSTRATE, FLEXIBLE DEVICE, ELECTRONICS DEVICE, AND PROCESS FOR PRODUCING THE SAME”), Japanese Application No. 2004-088853 (filed Mar. 25, 2004, the title of the invention: “FLEXIBLE SUBSTRATE, MULTILAYER FLEXIBLE SUBSTRATE, FLEXIBLE DEVICE, AND PROCESS FOR PRODUCING FLEXIBLE SUBSTRATE”), Japanese Application No. 2004-088854 (filed Mar. 25, 2004, the title of the invention: “FLEXIBLE SUBSTRATE, FLEXIBLE DEVICE, AND PROCESS FOR PRODUCING FLEXIBLE SUBSTRATE”), Japanese Application No. 2004-318887 (filed Nov. 2, 2004, the title of the invention: “PROCESS FOR PRODUCING FLEXIBLE SUBSTRATE, FLEXIBLE SUBSTRATE, FLEXIBLE DEVICE, AND CIRCUIT BOARD MODULE”), Japanese Application No. 2004-318888 (filed Nov. 2, 2004, the title of the invention: “PROCESS FOR PRODUCING FLEXIBLE SUBSTRATE, AND FLEXIBLE SUBSTRATE, FLEXIBLE DEVICE”), the disclosures of which are all incorporated herein by reference. 

1. A process for producing a flexible substrate comprising of a film, an insulating resin layer and a wiring pattern, said process comprising the steps of: (a) preparing a sheet member comprising, (i) the film, (ii) the insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face, and (iii) a front-sided wiring pattern embedded in said insulating resin layer formed on said front face of said film, and a rear-sided wiring pattern embedded in the insulating resin layer formed on said rear face of said film; and (b) pressing a part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern into the inside of said sheet member so that a part of said front-sided wiring pattern and a part of said rear-sided wiring pattern are jointed to each other to form a junction.
 2. The process according to claim 1, wherein said part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern is pressed by means of a needle-like member or a roll member with protrusions in said step (b).
 3. The process according to claim 1, wherein, in said step (b), a conductive member is disposed on a part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern and thereafter is pressed, so that said part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern is pressed into the inside of said sheet member, and thereby said part of said front-sided wiring pattern and said part of said rear-sided wiring pattern are jointed to each other.
 4. The process according to claim 1, further comprising the step of applying an ultrasonic wave to said junction.
 5. The process according to claim 1, wherein said sheet member is prepared in said step (a) by transferring a preliminary formed wiring pattern to the insulating resin layer.
 6. A flexible substrate comprising; a film, an insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face, and a front-sided wiring pattern embedded in the insulating resin layer formed on said front face of said film, and a rear-sided wiring pattern embedded in the insulating resin layer formed on said rear face of said film, wherein a part of at least one of said front-sided wiring pattern and said rear-sided wiring pattern is dented toward the inside of said flexible substrate so that a part of said front-sided wiring pattern and a part of said rear-sided wiring pattern are jointed to each other to form a junction.
 7. The flexible substrate according to claim 6, wherein a cross-sectional view of a wiring section composed of said part of said front-sided wiring pattern and said part of said rear-sided wiring pattern is “X” or “U” in shape.
 8. The flexible substrate according to claim 6, wherein said film is thinner than said insulating resin layer.
 9. The flexible substrate according to claim 8, wherein a ratio of an insulating resin layer thickness to a film thickness is 1.2 to
 6. 10. The flexible substrate according to claim 6, wherein said film is made of an aramid.
 11. The flexible substrate according to claim 6, wherein a concave portion in a surface of the wiring pattern, which portion is formed due to the dent, is filled with a conductive member so that the surface of said wiring pattern is flat.
 12. The flexible substrate according to claim 6, wherein said junction is treated by means of an ultrasonic wave.
 13. A multilayer flexible substrate in which a plurality of flexible substrates are laminated, wherein at least one of said flexible substrates is the flexible substrate according to claim
 6. 14. A process for producing a flexible substrate comprising of a sheet member and a substrate, said process comprising the steps of; (a₁) preparing the sheet member comprising, (i) a film, (ii) an insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face, and (iii) a wiring pattern embedded in the insulating resin layer formed on the front face of said film (a₂) preparing the substrate having a wiring pattern formed on a front face thereof, (b) stacking said sheet member on said substrate in such a manner that the insulating resin layer formed on said rear face of said film of said sheet member is contacted with said front face of said substrate, and thereafter a part of the wiring pattern of said sheet member is pressed toward said substrate so that said part of the wiring pattern of said sheet member and a part of the wiring pattern of said substrate are jointed to each other to form a junction.
 15. The process according to claim 14, wherein said part of the wiring pattern of said sheet member is pressed by means of a needle-like member or a roll member with protrusions in said step (b).
 16. The process according to claim 14, wherein, in said step (b), a conductive member is disposed on a part of the wiring pattern of said sheet member and thereafter is pressed, so that said part of the wiring pattern of said sheet member is pressed toward said substrate, and thereby said part of the wiring pattern of said sheet member and said part of the wiring pattern of said substrate are jointed to each other.
 17. The process according to claim 14, further comprising the step of applying an ultrasonic wave to said junction.
 18. A flexible substrate comprising of a sheet member and a substrate having a wiring pattern formed on a front face thereof, wherein said sheet member comprising a film, an insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face, and a wiring pattern embedded in the insulating resin layer formed on the front face of said film, said sheet member is stacked on said substrate in such a manner that the insulating resin layer formed on said rear face of said film of said sheet member is contacted with said front face of said substrate, a part of the wiring pattern of said sheet member is dented toward said substrate so that said part of the wiring pattern of said sheet member and a part of the wiring pattern of said substrate are jointed to each other to form a junction.
 19. The flexible substrate according to claim 18, wherein a cross-sectional view of said part of the wiring pattern of said sheet member is “U” in shape.
 20. The flexible substrate according to claim 18, wherein another wiring pattern is formed on a rear face of said substrate which face is opposite to said front face thereof, said sheet member is staked on each of said front face and said rear face of said substrate, a part of each wiring pattern of the sheet member stacked on said front face of said substrate and the sheet member stacked on said rear face of said substrate is dented toward said substrate, so that said part of each wiring pattern of the sheet member stacked on said front face of said substrate and the sheet member stacked on said rear face of said substrate is jointed with a part of each of the wiring pattern formed on said front face of said substrate and the wiring pattern formed on said rear face of said substrate.
 21. The flexible substrate according to claim 18, wherein said film is thinner than said insulating resin layer.
 22. The flexible substrate according to claim 21, wherein a ratio of an insulating resin layer thickness to a film thickness is 1.2 to
 6. 23. The flexible substrate according to claim 18, wherein said film is made of an aramid.
 24. The flexible substrate according to claim 18, wherein a concave portion in a surface of the wiring pattern, which portion is formed due to the dent, is filled with a conductive member so that the surface of said wiring pattern is flat.
 25. The flexible substrate according to claim 18, wherein said junction is treated by means of an ultrasonic wave.
 26. A multilayer flexible substrate in which a plurality of flexible substrates are laminated, wherein at least one of said flexible substrates is the flexible substrate according to claim
 18. 27. A flexible device, comprising: the multilayer flexible substrate according to claim 13, and a semiconductor chip mounted on a wiring pattern located at a surface of said multilayer flexible substrate.
 28. A flexible device, comprising: the multilayer flexible substrate according to claim 13, and an electronic component included within at least one of flexible substrates that constitute said multilayer flexible substrate.
 29. An electronics device, including the multilayer flexible substrate according to claim 13 as a circuit board.
 30. A flexible device, comprising: the multilayer flexible substrate according to claim 26, and a semiconductor chip mounted on a wiring pattern located at a surface of said multilayer flexible substrate.
 31. A flexible device, comprising: the multilayer flexible substrate according to claim 26, and an electronic component included within at least one of flexible substrates that constitute said multilayer flexible substrate.
 32. An electronics device, including the multilayer flexible substrate according to claim 26 as a circuit board. 